In a vivarium setting, small animals, such as mice, are typically confined in a metabolic cage with food and water for periods of time that can be up to 24 hours or longer. Confinement of the animal in the cage facilitates a prolonged study of the animal including collection of urine and feces for chemical and immunological analysis.
When confined in the cage, the animal walks or rests upon the cage's bottom surface, which is formed of interlinked wires separated by gaps to form a wire mesh. The wire mesh is quite uncomfortable and even painful for an animal to rest upon and/or walk upon for an extended period of time. The wire grid does not provide proper support for the animal's feet and the gaps between the wires is often large so that the animal can slip, as described and shown in more detail below.
This uncomfortable environment can negatively stress the animal. This can adversely affect the data collected during the animal's confinement and can cast doubt on data that results from the animal's confinement. Mice confined in metabolic cages display elevated levels of oxidative stress, hyperthermia and muscle catabolism. This is the case for all mice, because of their size and difficulty to navigate on the metal gridwork. However, mice that are severely obese, as is often the case in metabolic studies of diabetic mice have even more problems. The confinement of these obese animals in a metabolic cage to collect urine samples is especially painful as the large girth of the mice causes them to experience high levels of chafing and irritation from the wire mesh on the bottom of the cage.